Double metal cyanide complexes are well-known catalysts for polymerizing epoxides. The catalysts are extremely reactive, and can be used to make polyether polyols, even high molecular weight polyols, having low unsaturation.
Following epoxide polymerization, catalyst residues are preferably removed from polyether polyols because polyols containing catalyst residues tend to accumulate volatile impurities during storage. In addition, residual polyol catalysts can have an unintended catalytic or poisoning effect during formulation of the polyols in polyurethanes. Methods of removing conventional basic catalysts from polyols (such as, for example, aqueous extraction or adsorption with magnesium silicate) are often ineffective for removing DMC catalyst residues from polyols. Consequently, several methods have been developed to address the particular problem of removing DMC catalysts from polyols.
U.S. Pat. Nos. 4,355,188 and 4,721,818, for example, teach methods for treating polyols made with a DMC catalyst. The polyol, which contains DMC catalyst residues, is heated with an excess of an alkali metal compound (metal, hydride, hydroxide), which deactivates the DMC catalyst, and converts a substantial proportion of the hydroxyl end groups of the polyol to alkoxide end groups. Ethylene oxide is then added to end-cap the polyol to give it primary hydroxyl end groups. The polyol product is then diluted with isopropyl alcohol and is ion-exchanged to remove DMC and alkali metal catalyst residues, or is treated with magnesium silicate and filtered to remove the catalysts. As Katz et al. suggest in U.S. Pat. No. 5,099,075, the method of U.S. Pat. No. 4,355,188 is not completely satisfactory for purifying all-propylene oxide-based polyols. Both techniques appear to be most useful when an EO end-capping step is included.
U.S. Pat. No. 4,877,906 teaches a multi-step method for purifying polyols made with DMC catalysts. First, the polyol is treated with an alkali metal compound in an amount sufficient to convert the DMC compound to an insoluble species, and at least some of the polyol hydroxyl groups to alkoxide groups. Second, the polyol is filtered. Third, the polyol is heated with a phosphorus-containing acid such as hypophosphorous acid. Finally, the product is filtered again to recover the purified polyol. The comparative examples in Table 3 of the reference indicate that treatment with KOH or NaOH at 1400-2700 ppm alone is ineffective in removing residual zinc from an EO-capped polyether polyol made with a DMC catalyst.
One technique applicable to all-PO polyols is described in U.S. Pat. No. 5,010,047. The method involves diluting the polyol with a nonpolar organic solvent such as hexanes, then filtering at room temperature using a filter aid such as diatomaceous earth. This method is impractical for large-scale use because large volumes of solvent must be combined with the polyol, stripped, and recovered. A more practical method would eliminate the need for a solvent.
U.S. Pat. No. 5,099,075 discloses a method for removing DMC catalyst residues from polyether polyols. The catalyst-containing polyol is treated with an oxygen-containing gas, oxidizing acid, or peroxide to convert the DMC catalyst to an insoluble species. Side products from over-oxidation might reasonably be expected from the use of such strong oxidants with polyether polyols because polyols are susceptible to oxidation.
Japanese Patent Appl. Kokai No. 4-214722 teaches a method for making polyether polyols using DMC catalysts. The reference teaches (Practical Example 1) a catalyst-removal method in which a poly(oxypropylene) polyol is heated in the presence of 6700 ppm of potassium methoxide to deactivate the DMC catalyst. The mixture is heated with magnesium silicate and water at 90.degree. C., is filtered in the presence of the water, and is then stripped to remove water. The comparative examples show that filtration of the polyols is slower and catalyst removal is less complete when water is not included in the process.
Most treatment methods use magnesium silicate, diatomaceous earth, or mixtures of these to assist in removal of DMC catalyst residues. The methods commonly use from 2-10 wt. % of solids. For a lab-scale process, this amount of solids creates no substantial problem. However, when the treatment is performed on a large scale, it becomes imperative to minimize the amount of solids used to reduce material and waste-disposal costs. In addition, substantial polyol losses are incurred in the resulting filter cakes.
Improved methods for removing DMC catalyst residues from polyether polyols, particularly poly(oxypropylene) polyols, are needed. Preferably, multiple treatment steps would be avoided. A preferred method would not use solvents, would overcome any need to use strong oxidants, and would give consistent, reliable removal of DMC catalyst residues. The method would preferably minimize the amounts of treating reagents needed to achieve complete removal of the DMC catalyst residues to reduce reagent, treatment, and disposal costs. A method that would eliminate the need for an adsorption step with magnesium silicate would also be valuable.